The Zeeman-sensitive emission lines of Mg I at 12 micron in Procyon

Emission-lines of magnesium at 12 microns have been observed in the spectrum of Procyon. We reproduce the observed, disk-averaged line flux from Procyon (as well as the observed intensity profiles from the Sun) by calculating the line formation, relaxing the assumption of Local Thermodynamic Equilibrium. We find that the lines in Procyon are formed in the photosphere in the same manner as the solar lines. We discuss our modeling of these Rydberg lines and evaluate, among other things, the importance of the ionizing flux and updated model-atom parameters. The lines are of large diagnostic value for measurements of stellar magnetic-fields through their Zeeman splitting. We have not, however, detected splitting of the MgI lines in Procyon. Using simple arguments, we believe we would have detected a magnetic field, had it been of a strength larger than approximately 800 Gauss covering more than a quarter of the surface. We discuss the prospects for future use of the Zeeman-sensitive, mid-infrared, MgI emission lines as a diagnostic tool for stellar magnetic fields.